Field of the Invention
The present invention relates to a machining curve creating apparatus and a machining curve creating machining curve creating method in which a tool path allowing a smooth machined surface to be provided during machining with a numerically controlled machine tool is determined using a numerical control apparatus controlling the machine tool or an information processing apparatus such as a personal computer.
Description of the Related Art
A method is widely known for determining a curve interpolating a given point sequence or a curve passing by each of the points (Les Piegl, Wayne Tiller, The NURBS Book, Springer-Verlag, and I. J. Schoenberg, Spline functions and the problem of graduation, Proceedings of the National Academy of Science of the U.S.A., 52(1964)). For machining with a numerically controlled machine tool, the method is utilized to convert a tool path given as a point sequence into a curve in order to provide a smooth machined surface and to reduce a machining time (Japanese Patent Lid-Open No. 2013-171376, Japanese Patent Lid-Open No. 2007-293478, Japanese Patent Lid-Open No. 2006-309645, Japanese Patent Lid-Open No. 2005-182437, and Japanese Patent Lid-Open No. 2004-078516).
In general, the length of a machining program varies depending on a workpiece, and time-consuming machining involves a very large number of points providing a tool path. Thus, completing curving during one process is difficult, and a method is adopted in which the point sequence is divided into pieces to allow the curve to be generated step by step. Furthermore, this method is inevitably adopted when curve generation and machining are performed in parallel.
The division of the point sequence is normally performed using a method of taking a predetermined number of points starting with the leading point. When the point sequence includes a bent portion, the bent portion is set to be a dividing point of the point sequence even if the number of points up to the bent portion is smaller than the predetermined number. Curves generated from groups of points resulting from the division are expressed in the form of parametric curves. An expression form such as a B spline curve or a NURBS curve is adopted which is suitable for the application.
Conventional machining curve creating focuses on a method for generating curves from a point sequence resulting from division and does not particularly take into account the shape of the entire tool path resulting from joining of generated curves. As a result, disadvantageously, satisfactory curves fail to be generated when adjacent curves are desired to be similar in shape as in the case of a forward path and a backward path of a reciprocating machining path.
In general, in a machining program output by a CAM, the positions of points expressing a machine path may deviate slightly from a target path in connection with a calculation method or the like. Furthermore, the interval between the points commonly varies. In the reciprocating path, points providing the forward path do not always correspond to points providing the backward path. In the point sequence dividing method of taking a predetermined number of points, the spatial distribution of the divisions of the point sequence varies between the forward path and the backward path. The different spatial distributions of the point sequence in turn vary the shape of the generated curve, leading to step-like differences between the curve of the forward path and the curve of the backward path. A similar problem occurs in the case of a spiral tool path that is placed around the circumference of a workpiece. The cause of the problem is that the method of taking a predetermined number of points fails to control the spatial distribution of point sequences. This indicates the need of a dividing method allowing the spatial distributions to be aligned with each other.